1. Field of the Invention
This invention relates to a method for preparation of a functional film, particularly a multi-layer structure film which is useful for uses in electronic devices such as semiconductor devices, photosensitive devices for electrophotography, optical input sensor devices for optical image inputting devices, etc.
2. Related Background Art
Devices by use of amorphous or polycrystalline deposited films of multi-layer structures such as semiconductor films, insulating films, photoconductive film, magnetic films or metal films can be expected to have physical characteristics or uses not desired for deposited films of single layer structure and therefore have been studied aggressively in recent years. Particularly from the standpoint of large area devices, multi-layer structure films having two or more kinds of amorphous layers laminated are attracting interest.
For example, investigations have been made to prepare multi-layer structure films having amorphous silicon (a-Si) layer and amorphous silicon carbide (a-SiC) layer, or amorphous silicon layer and amorphous silicon germanium (a-SiGe) layer laminated alternately according to the chemical vapor deposition method such as the plasma CVD method or the optical CVD method, and they are contemplated to be applied for solar battery or other devices.
Particularly, formation of a deposited film according to the plasma CVD method, while there is involved the problem such that its reaction mechanism has not a few ambiguous points, is now accepted as the best method in view of productivity and characteristics of the film and has been widely applied for production of solar battery, electrophotographic photosensitive member, etc. However, there are a large number of parameters for formation of a deposited film (for example, substrate temperature, flow rate and flow rate ratio of the introduced gases, pressure during formation, high frequency power, electrode structure, structure of the reaction vessel, speed of evacuation, plasma generating system, etc.). By use of a combination of such a large number of parameters, the plasma may sometimes become unstable state, whereby marked deleterious influences may be exerted frequently on the deposited film formed.
Particularly, when forming a multi-layer structure film according to the plasma CVD method, it is necessary to control discharging or gas introduction every time when the kind of the layer is changed.
However, if the amount of the gas introduced is changed to a great extent, the pressure within the reaction space will be changed, whereby the state of discharging becomes unstable to affect badly the structure and the characteristics of the film. Accordingly, if a multi-layer structure is formed only by controlling the amount of the gas introduced with discharging being kept on as such as practiced in the prior art, it is difficult to adjust the conditions so that films with good characteristics may be constantly obtained.
On the other hand, the method in which deposited films are formed by controlling discharging, namely exchanging the gases with discharging being turned off and turned on again after the pressure has reached equilibrium, will take an extremely long time to be disadvantageously poor in productivity. Also, in plasma discharging, discharging is not generally stabilized immediately after discharging is turned on, whereby multi-layer films are obtained which have interfaces of bad characteristics.
For example, when an a-SiGe layer with an optical band gap of 1.5 eV is formed by glow discharging decomposition of SiH.sub.4 gas and GeH.sub.4 gas, the quantity
ratio of SiH.sub.4 and GeH.sub.4 is made generally 2:1. Accordingly, if the flow rate of GeH.sub.4 is controlled for the purpose of preparing a multi-layer structure film of a-SiH layer and a-SiGe:H layer, the pressure within the film forming space receives fluctuation of about 30%. For this reason, the discharging state will change greatly, whereby it is difficult to prepare a multi-layer structure film with good characteristics.